1. Field of the Invention
The present invention relates to an ultrasonic vibration sieving apparatus and a method for purifying carbon black which contains solid impurities by removing the solid impurities therefrom by using the ultrasonic vibration sieving apparatus. More specifically, the present invention relates to an ultrasonic vibration sieving apparatus and a process for purifying carbon black by removing water insoluble and water soluble impurities contained in the carbon black such as furnace black produced by an oil furnace type reaction furnace, by-product carbon black formed in a partial oxidation furnace of a heavy oil and pyrolytic carbon black formed by the pyrolysis of a gas.
2. Description of the Prior Art
Carbon black is commonly widely used as a pigment for a resin, paint or ink, as a reinforcing material for rubber, or as an electric conductivity-imparting material for a resin, rubber or ink.
When carbon black is to be incorporated directly into a thermoplastic resin or rubber, it is common to employ a Bumbury's mixer, a mixing roll mill, a Henschel mixer, a pressure kneader, a screw type extruder or the like. For the incorporation into a paint or ink, carbon black is added at the same time as, or after a thermoplastic or thermosetting resin or rubber is dissolved in a solvent, and the mixture is blended in a sand mill or a ball mill.
In recent years, it has been common to employ a mass-coloring method in which carbon black is incorporated to a monomer and thoroughly dispersed therein, followed by filming or spinning.
However, a number of troubles have been caused by solid impurities or foreign matters contained in the carbon black. For instance, during the kneading or molding of the resin or rubber by an extruder, such solid impurities or foreign matters cause clogging of the screen in front of the breaker plate of the extruder. Unusual wearing off is caused at the portion of the spinning machine which is in contact with filaments span after the mass-coloring, whereby the production efficiency substantially deteriorates. Further, small protrusions attributable to the solid impurities are likely to form on the surface of an extrusion-molded product, an inflation molded product or a press molded product, or scars are likely to be formed on the suface of a press-molding die, or breakage of filaments is
likely to be caused during the spinning after the mass-coloring, whereby the commercial values of the products will be substantially impaired.
If carbon black contains water soluble solid impurities composed of an alkali metal salt or an alkaline earth metal salt, there will be a serious trouble such that when such carbon black is incorporated to a resin and applied to the surface of a high density recording material, the nature of the treated surface of the high density recording information material will be changed.
In order to separate and remove such solid impurities from carbon black, it has been common to employ a method wherein a mechanical separation means such as a sieve or cyclone is used depending upon the type and the particle size of the impurities, or a method wherein carbon black is formed into an aqueous slurry, and then sieved by means of a mechanical vibration sieve, as disclosed in Japanese Unexamined Patent Publication No. 11963/1981.
However, according to the above-mentioned methods, the ability for the removal of the solid impurities is limited to a level of from 10.sup.-2 to 10.sup.-3 % at best, at a particle size of 44 .mu.m on the sieve. With such a removal rate, it is practically impossible to prevent the clogging of the screen of the extruder or to prevent the deterioration of the surface texture of the molded products. It is totally impossible to prevent the deformation of the treated surface of a shaped resin product such as the information material due to the impurities composed of an alkali metal salt or an alkaline earth metal salt.
Further, in the case of the latter method, coagulated carbon black will remain on the sieve, and it will be necessary to remove the coagulated carbon black, whereby it will be difficult to conduct the treatment in a continuous process.